PAPI and VASI seem to be very similar in the the information they provide - i.e. are you on the glide slope or off it. Is there a functional difference between the two? If there is a difference, and one is superior to the other, is there a construction cost savings where an airport might chose one over the other?

2 Answers
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A PAPI and a VASI are very similar in the the information they provide. The only functional differences between the VASI and PAPI is that the VASI has the red over the white, the PAPI the white actually goes to the right of the red, and the PAPI offers higher precision (or more glideslopes depending how you look at it a larger airliner with a high cockpit may elect to fly a slightly higher glidepath). The concept is the same though.

A VASI looks like this:

Usage:

As the saying goes,

Red over White, you're alright. (on glidepath)

Red over Red, you're dead. (too low)

White over White, you're out-of-sight (too high)

A PAPI like this:

A regular VASI only offers one glide slope and is designed for an aircraft where the cockpit isn't so high up. However, there is a such thing as two-light PAPI, and a three-bar VASI. So either of them can really be tailored to fit the costs and types of aircraft flying to that airport.

In the case of a four-bar PAPI it is higher precision. Since the PAPI systems uses a narrower beam of light you must fly the glide path more precisely than the VASI to stay on the beam. The PAPI, with its extra lights, forewarns you when you are drifting from the desired glide path.

So one red light would indicate slightly above glide slope, two and two would indicate the normal glide slope, and three red lights would indicate slightly below the glide slope on a 4-bar PAPI. A Three bar VASI works in similar fashion except there are only two glide paths with two reds being the lower, two whites the higher.

The PAPI is functionally different from the VASI in terms of precision (hence the name!).

A Precision Approach Path Indicator (PAPI) uses four sets of two lamps, placed next to each other to provide the pilot with 5 distinct levels of precision (r = red; w = white):

r r r r - below glideslope

r r r w - slightly below glideslope

r r w w - on glideslope

r w w w - slightly above glideslope

w w w w - above glideslope

There's also some degree of fading between red and white.

A typical Visual Approach Slope Indicator (VASI) uses four sets of two lamps in a box formation (two on top, two on bottom) to provide something more like 3 distinct sets of information:

r rr r - below glideslope

r rw w - on glideslope

w ww w - above glideslope

The more vertical placement of the VASI lights is the origin of the mnemonic "white over white, you're high as a kite; red over white, you're all right; Red over red, you're dead".

Operation

As far as operation and installation goes, I'm under the impression that both use the same two-lamp modules, configured with lenses, reflectors, and filters. Presumably the cost to install a VASI is lower, which is why they're much more common; alignment and maintenance is also likely to be cheaper as well.

Furthermore, if one set of lamps in a PAPI is out of service, the entire unit is unusable. If a set of lamps in a VASI is out, the redundancy of the system means that it still can provide some information (albeit information that should be used with extreme caution).